Field of the Invention
The present invention relates to an inductive flow probe for measuring the local flow velocity of a stream of liquid metal.
An inductive flow probe comprises a tube, for insertion into a stream of liquid metal, which is closed at one end and contains at least one permanent magnet disposed within the tube. A magnetic field having a direction transverse to the major flow direction of the liquid metal is generated by the magnet, and a pair of thermoelements is positioned within the magnetic field. An evaluation circuit is connected to the probe for determining the velocity of the liquid metal. One such system is disclosed in U.S. Pat. No. 4,145,924.
Probes of this type are used to measure local velocities in liquid metals as, for example, in the circulation of a sodium cooled nuclear reactor, particularly in the area of the fuel elements so as to constantly monitor the stream of liquid metal and avoid local overheating.
A permanent-magnet velocity measuring probe for liquid metals is shown in FIGS. 1 and 1a, which comprises a tube 5 and permanent magnet 6. This measuring probe 5 operates according to Faraday's principle of induction. That is, if an electrically conductive liquid flows through a magnetic field in a flow direction (indicated by arrows A) which is different from the direction of the magnetic field, an electric field is generated in the liquid. A measured difference in electrical potential E.sub.12 between two points 1 and 2 of the electric field is proportional to the flow velocity V, and the following equation applies: EQU E.sub.12 =C.sub.1 (B.multidot.V)
where C.sub.1 is a proportionality constant which depends on the properties of the materials involved and on the geometry of the arrangement and which, in a specific case, must be determined by calibration.
The calibration can also be made after the probe has been installed at its point of use if, instead of one permanent magnet 6, two or more permanent magnets 6 and 6' are arranged in the direction of flow as shown in FIG. 2. In this arrangement, the transit time .tau..sub.m of any fluctuations in the velocity between the two magnets 6 and 6' may be determined by a correlation of the associated probe signals at points 1, 2, 3 and 4 and from this, if the distance between the magnets 6 and 6' is known, the average flow velocity can be determined.
The embodiment shown in FIG. 2 which employs two or more magnets 6 and 6' is advantageous in that changes in the magnetic field intensity as a result of temperature influences, irradiation or aging, and thus changes in the measured voltage can be eliminated by using the described calibration method.
Details of the structure and operation of such probes were published in a report by Kernforschungszentrum Karlsruhe (St. Muller, G. Thun, "Permanentmagnetische Durchflussme.beta.sonde fur flussige Metalle" which translates to "Permanent-Magnet Flow Measuring Probe For Liquid Metals"). In the permanent magnetic flow velocity measuring probe described in this article, two steel wires are employed as the measuring electrodes to pick up locally the potential difference induced by the flow velocity.
One of the problems with the probes of the prior art is that an assumption is necessary. With the probes of the prior art, one must assume that no temperature difference exists between the measuring electrodes, i.e. no temperature gradients exist in the stream, so that the probe signal changes in proportion to the flow velocity. If this assumption is not met, such as in streams flowing through channels with heated walls or in streams that flow upwardly, a temperature component is superimposed on the probe signal in addition to the velocity component. This temperature component corresponds to the difference in thermoelectric potential between the two ends of the measuring electrodes and may be greater by some multiple than the velocity signal. Since the temperature difference between the two pickup points cannot be measured, the temperature component in this type of probe configuration cannot be compensated.
One solution for the previously noted problem is discussed in a dissertation by T. von Weissenfluh, entitled "Turbulenter Warmetransport in flussigem Natrium" which translates to "Turbulent Heat Transport in Liquid Sodium", ETH Zurich (1984), Diss. ETH No. 7464. Von Weissenfluh discloses a permanent-magnet velocity measuring device in which the temperature difference between the potential pickup points can be measured in addition to the velocity component by using pickup electrodes comprising open Cromel/Alumel thermoelements. Although this makes it possible in principle to obtain temperature compensation for the probe signal, it requires an accurate knowledge of the Seebeck coefficients of the measuring electrode material (i.e., Cromel and Alumel) and of the liquid metal (e.g. sodium). Since Seebeck coefficients depend on absolute temperature, this dependency must also be considered when obtaining a temperature compensation value for the probe signal. Thus, a problem with this probe is that continuous compensation of the probe signal requires, in addition to measuring the temperature difference between the measuring electrodes, a constant indirect measurement of a physical parameter i.e., the Seebeck coefficients of at least the electrode material and the liquid metal.